The invention relates to a thin-film capacitor comprising an insulating substrate which is provided with at least two inner electrode layers which are separated from each other by means of a dielectric layer, the substrate being also provided with two end contacts each electroconductively contacting one of the inner electrode layers.
A thin-film capacitor of the type mentioned in the opening paragraph is known per se, for example, from U.S. Pat. No. 4,453,199. More particularly, said patent describes a thin-film capacitor which is provided with an insulating substrate which is preferably made of glass. By means of thin-film techniques, said substrate is successively provided with a sub-layer of phosphor-doped silicon dioxide, a first inner electrode layer of aluminium, a dielectric layer of silicon dioxide, a second inner electrode layer of aluminium and an overlayer of silicon dioxide. The known capacitor is further provided with two sputtered end contacts. Via one of its ends, each inner electrode electrically contacts one of the end contacts of the capacitor.
It has been found that the known thin-film capacitor has a major drawback. It has been found that when mounting such a thin-film capacitor on a printed circuit board, breakdown may occur due to accumulation of electric charge. This phenomenon is known as electrostatic discharge (ESD). This problem notably occurs in capacitors having a comparatively small thickness of the dielectric layer. This drawback is particularly a problem when the capacitor is mounted in areas in which large electric charges can easily build up. The current trend of using capacitors with an ever-decreasing thickness of the dielectric layer, for example to values of 100 nm and less, leads to said problem becoming more and more pressing.
It is to be noted that said ESD problem does not occur in thick-film capacitors and in ceramic multilayer capacitors, because they have a relatively thick dielectric layer.
It is an object of the invention to alleviate this drawback. More particularly, it is an object of the invention to provide a thin-film capacitor in which the occurrence of breakdown due to build-up of electric charge during mounting is reduced considerably.
These and other objects of the invention are achieved with a thin-film capacitor comprising an insulating substrate which is provided with at least two inner electrode layers which are separated from each other by means of a dielectric layer, and with two end contacts each electroconductively contacting one of the two inner electrode layers, which capacitor according to the invention is characterized in that the capacitor is also provided with breakable electrically conducting connection means which connect both end contacts electrically.
By using said measure, the problem described above is strongly alleviated. The presence of breakable electrically conducting connection means between the end contacts prevents damage to the capacitor caused by electric charge when mounting the capacitor on a printed circuit board. By using the construction according to the invention, such a charge is directly neutralized via the electrically conducting connection means. After fixing the capacitor on the printed circuit board, the electrically conducting connection means can be removed, for example by means of a laser operation or by applying a controlled burst of current. This interrupts the electrically conducting contact between the end contacts.
A preferred embodiment of the film-film capacitor according to the invention is characterized in that the breakable electrically conducting connection means comprise a metal layer. Due to its high electrical conductivity, such a metal layer may immediately deplete possibly occurring charge of the capacitor described.
A further favorable embodiment of the capacitor according to the invention is characterized in that the metal layer forms part of an electrode layer. This embodiment provides major advantages in the mass manufacture of the thin-film capacitor. In this case, an extra process step of providing the metal layer is not necessary, but this layer can be provided simultaneously with the electrode layer to which it is connected. It is to be noted that it is, in principle, possible to provide the metal layer, possibly in a separate process step, as a separate layer in addition to the electrode layers.
An advantageous embodiment of the thin-film capacitor according to the invention is characterized in that the metal layer consists substantially of aluminium. Practice has proved that a metal layer of substantially aluminium can be interrupted in a simple way by using a laser operation.
Another interesting embodiment of the thin-film capacitor according to the invention is characterized in that the metal layer consists substantially of a metal which dissolves in solder. This has the important advantage that the metal layer is interrupted during soldering the capacitor on a PCB. As a result, a separate step of interrupting this layer is no longer necessary. Suitable metals for such a layer are Au and particularly Ag.
Another favorable embodiment of the thin-film capacitor according to the invention is characterized in that the electrically conducting connection means comprise a layer of electrically conducting metal compounds which, under the influence of a thermal treatment, are converted into non-conducting metal compounds. In principle, the thermal treatment may take place with the aid of a laser. However, when the capacitors are provided by means of the so-called reflow soldering technique, said conversion takes place simultaneously when soldering the capacitors on a PCB. In that case, a separate thermal treatment is no longer necessary. The class of electrically conducting metal (oxy)nitrides has been found to be very suitable for use in this embodiment. Under the influence of heat, these compounds are converted into the corresponding metal oxides which are electrically non-conducting. Interesting examples are TiN, TaN, TiOxNy and TaOxNy, in which 0 less than x less than 1, 0 less than y less than 1 and x+y=1).
A further embodiment of the thin-film capacitor according to the invention is characterized in that the electrically conducting connection means comprise a layer of a low melting point metal alloy having a high surface tension. When soldering the capacitor on a PCB, the metal alloy melts. Due to the high surface tension, there will be coagulation so that the layer becomes electrically non-conducting. Suitable metal alloys for this embodiment are PbxSny, SnxBiy, SnxAgy, SnxCuy, AgxSby and SnxZny (in which 0 less than x less than 1, 0 less than y less than 1 and x+y=1).
A further interesting embodiment of the thin-film capacitor according to the invention is characterized in that the capacitor also comprises one or more floating electrode layers which do not electroconductively contact the end contacts. The advantageous effect of the breakable electrically conducting connection means between both end contacts is obtained also in this type of thin-film capacitor.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.